Serial communication is used for communication between a master device and a plurality of slave devices. For writing data in registers in the slave devices by the master device, the master device sends a pair of address data and serial data to the slave devices. For writing the data in the plurality of slave devices, it is necessary for the master device to designate an address for each of the registers in the slave devices and write the data. Therefore, as the number of registers in the slave devices increases, it takes longer to write all the data in the registers.